Oxygen-rich tetrahedral surface phase on high-temperature rutile $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ single crystals

Vanadium dioxide undergoes a metal-insulator transition from an insulating (monoclinic) to a metallic (tetragonal) phase close to room temperature, which makes it a promising functional material for many applications, e.g., as chemical sensors. Not much is known about its surface and interface properties, although these are critical in many applications. In this paper, we present an atomic-scale investigation of the tetragonal rutile $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ single-crystal surface and report results obtained with scanning tunneling microscopy, low-energy electron diffraction, and x-ray photoelectron spectroscopy, supported by density-functional-theory-based calculations. The surface reconstructs into an oxygen-rich (2 × 2) superstructure that coexists with small patches of the underlying unreconstructed $\left(110\right)\text{−}\left(1\times 1\right)$ surface when the crystal is annealed $>600{}^{\circ }\mathrm{C}$. The best structural model for the (2 × 2) surface termination, conceptually derived from a vanadium pentoxide (001) monolayer, consists of rings of corner-shared tetrahedra. Over a wide range of oxygen chemical potentials, this reconstruction is more stable than the unreconstructed (110) surface and models proposed in the literature.

Figure 1

The $\mathrm{V}{\mathrm{O}}_2$ single crystals. (a) A selection of large, as-grown crystals exposing the flat and shiny side. Small crystals are usually needle shaped, like the rightmost crystal in the picture. (b) A crystal mounted for scanning tunneling microscopy (STM) measurements, held tightly by a tantalum spring and gold foil. (c) A crystal mounted in a heated molybdenum sample holder for x-ray photoelectron spectroscopy (XPS) measurements, with a thermocouple on top and held in place by an additional Chromel wire.

Figure 2

Determination of the surface orientation. (a) and (b) Crystal fragment investigated with x-ray diffraction (XRD) for temperatures below and above the metal-to-insulator transition (MIT), respectively. The experimentally obtained Miller indices of several crystallographic planes are indicated for each case. (c) Laue back-reflection of the monoclinic phase at room temperature with the sample rotated slightly out of normal incidence. (d) The same pattern superimposed by a fit.

Figure 3

Low-energy electron diffraction (LEED) pattern of the rutile phase at 200 °C taken at two different electron energies. The (1 × 1) pattern (solid orange rectangle) of a ${\left(110\right)}_{\mathrm{T}}$ termination and the (2 × 2) superstructure (dashed white rectangle) are indicated. Note that some reflections of the superstructure are not present in the image taken at 102 eV (white circles). Weak satellite spots are due to mosaicity.

Figure 4

The $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ surface after annealing at 560 °C, showing only the (1 × 1) surface without any superstructures in empty-states scanning tunneling microscopy (STM).

Figure 5

Scanning tunneling microscopy (STM) measurements on two different $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ crystals acquired at 80 °C. (a) Crystal with step edges along the [001] direction showing the unreconstructed $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}\text{−}\left(1\times 1\right)$ surface, narrow stripes of the (2 × 2), and patches of the c(4 × 2) phase on top of the (2 × 2). (b) Crystal with an almost complete overlayer of the (2 × 2) phase; holes in the (2 × 2) layer reveal $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}\text{−}\left(1\times 1\right)$.

Figure 6

Scanning tunneling microscopy (STM) contrasts and details of the (2 × 2) row structure. White markers in the top part of the panels indicate the positions of the wide spacing between the double rows. Comparison of (a) with (b) shows that contrast does not always emphasize the wide spacing as the main depression. Additional features situated in the wide sites are marked by white arrows. (a) The alignment of the wide spacing with respect to the bright rows of the $\mathrm{V}{\mathrm{O}}_2$ surface visible inside the holes is indicated. (b) Distinct double-row structure with domain boundaries (displacements within the double row along [100]), resulting in lines with zigzag (Z) structure, in contrast to the usual rectangular (R) arrangement.

Figure 7

Alignment of the (2 × 2) structure with respect to the $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}\text{−}\left(1\times 1\right)$. The scanning tunneling microscopy (STM) image of panel (b) is the same as in (a) with a high-pass filter applied to the (1 × 1) region. The grids in (b) are centered on the (1 × 1) rows and offset by $\frac{1}{2}\left[001\right]$ with respect to the maxima of these rows). Further explanations are given in the text.

Figure 8

Temperature-dependent x-ray photoelectron spectroscopy (XPS) core-level spectroscopy of a $\mathrm{V}{\mathrm{O}}_2$ single crystal. (a) and (b) The $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ phase after annealing in ultrahigh vacuum (UHV) at different temperatures. The surface was sputtered before each annealing step. All spectra were acquired at 200 °C in (a) grazing emission and (b) normal emission. (c) Normal and grazing emission spectra obtained on the monoclinic phase at 55 °C, after annealing at 650 °C [green curves in (a) and (b)]. All spectra were shifted to obtain a common O $1s$ binding energy of 530 eV.

Figure 9

Side views and simulated scanning tunneling microscopy (STM) images (empty states) of (a) and (c) the $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ and (b) and (d) the $\mathrm{V}{\mathrm{O}}_2{\left(011\right)}_{\mathrm{T}}$ bulk terminations using the Tersoff-Hamann approximation.

Figure 10

Buckled superstructure on $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ and the corresponding calculated scanning tunneling microscopy (STM) image. The structure was obtained by simulated annealing and relaxed using the SCAN functional. Bright spots in the simulated STM image are formed by the topmost oxygen row. Note that all oxygen atoms have the same height.

Figure 11

Conceptual steps toward the $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ ring terminations. (a) A vanadium pentoxide (001) monolayer. The yellow arrows mark the displacement direction of the vanadium atoms, which causes the transformation to the tetrahedral ring superstructure that fits the $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}\text{−}\left(2\times 2\right)$ supercell, shown in (b). (c) and (d) Side views of two possible connections of this ring structure with the underlying $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ lattice, resulting in an overall stoichiometry of ${\mathrm{V}}_4{\mathrm{O}}_{13}$. The purely tetrahedral ring termination (c) contains in the subsurface layer $\mathrm{V}=\mathrm{O}$ vanadyl bonds that can be subsequently removed (oxygen atoms colored in orange). The difference between the structures in (c) and (d) is a shift along the [001] direction as pointed out with the yellow arrow in (d), changing the coordination geometry of marked vanadium atoms from tetrahedra to square pyramids. The configuration in (d) is lower in energy than the structure in (c). (e) A calculated scanning tunneling microscopy (STM) image of the ring structure in (d). (f) and (g) Side and top views of another ring termination that can be derived from (b). The calculated STM image of this structure is shown in (h).

Figure 12

Calculated surface free energies (SCAN functional) of the $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ surface as a function of the oxygen chemical potential. Shown are the buckled $\mathrm{V}{\mathrm{O}}_2{\left(110\right)}_{\mathrm{T}}$ termination [from Fig. 10, black line], oxygen adsorption on the buckled ${\left(110\right)}_{\mathrm{T}}$ surface (green lines), and the ring terminations from Figs. 11 (blue line), 11 (orange line), and S8(a) and S8(b) in the Supplemental Material [32] (dashed lines). Gray dashed lines represent the reduced ring terminations with ${\mathrm{V}}_4{\mathrm{O}}_{12}$ and ${\mathrm{V}}_4{\mathrm{O}}_{11}$ surface stoichiometry. They were obtained from the ${\mathrm{V}}_4{\mathrm{O}}_{13}$ structure in Fig. 11 by subsequently removing the orange oxygen atoms from the subsurface layer, leading to ${\mathrm{V}}_4{\mathrm{O}}_{12}$ and ${\mathrm{V}}_4{\mathrm{O}}_{11}$ surface stoichiometry. The solid black vertical line represents the experimental stability limit of $\mathrm{V}{\mathrm{O}}_2$ with respect to the vanadium pentoxide phase.

Figure 13

The (SCAN) projected density of states (pDOS) onto vanadium and oxygen atomic orbitals in the rutile $\mathrm{V}{\mathrm{O}}_2$ and ${\mathrm{V}}_2{\mathrm{O}}_5$ bulk phases compared with the ${\mathrm{V}}_4{\mathrm{O}}_{13}$ ring termination. The DOS is given states per eV and colored in red and blue for O and V, respectively. The bands are aligned with respect to the upper edge of the O $2p$ band. The black vertical line denotes the Fermi level, which is determined by the $\mathrm{V}{\mathrm{O}}_2$ substrate in the case of the ring termination and drawn at the valence band maximum for ${\mathrm{V}}_2{\mathrm{O}}_5$.